Process for the preparation of thermally stable polyoxymethylene copolymers

ABSTRACT

A process for the preparation of polyoxymethylene copolymers, wherein 1,3,5-trioxane is polymerized with generally known comonomers in the presence of a strong protonic acid initiator and in the presence of a formaldehyde dialkyl acetal, and wherein the initiator is dissolved in the formaldehyde dialkyl acetal before admixing to the trioxane and the comonomers.

Process for the preparation of thermally stable polyoxymethylenecopolymers

The present invention relates to a process for the preparation ofthermally stable polyoxymethylene (POM) copolymers wherein the initiatoris distributed within the monomers through prior dissolution in aformaldehyde dialkylacetal.

Thermoplastic molding materials of POM homopolymers and copolymers havelong been frequently used as versatile materials of construction,particularly in engineering and manufacturing. In many cases they can beused as a substitute for metals on account of their outstandingmechanical properties, such as high rigidity, hardness and strength andthe fact that it is possible to produce moldings and molded parts tostrict tolerance limits, and their good resistance to many chemicals.

It is known that, by copolymerizing trioxane with cyclic ethers orcyclic acetals, copolymers can be obtained in which the sequence of the—CH₂—O— groups is interrupted by randomly distributed comonomer unitssuch as —CH₂CH₂—O—, —(CH₂)₄—O— or —CH₂—CH₂—O—CH₂—CH₂—O— (G. W. Becker/D.Braun, Kunststoff-Handbuch, Vol. 3/1, p. 303, Munich-Vienna, 1992). Thecomonomers are normally used in a weight proportion of 0.2 to 20%.Suitable initiators used in the present invention are strong protonicacids selected from the group consisting of trifluoromethanesulfonicacid and anhydrides, pentafluoroethylsulfonic acid and anhydrides,heptafluoropropylsulfonic acid and anhydrides, nonafluorobutyl sulfonicacid and anhydrides, perfluoroheptylsulfonic acid and anhydrides, andmixtures thereof Suitable initiators are also Lewis acids selected fromthe group consisting of phosphorus pentafluoride, silicon tetrafluoride,boron trifluoride, boron trifluoride etherates, tintetrachloride,arsenic pentafluoride, triphenylmethyl hexafluorophosphate, and mixturesthereof

At the end of the polymerization reaction the crude POM polymer stillcontains a certain amount of unconverted monomers and unstable terminalswhich have to be eliminated to stabilize the final product.

In order to be able to form such a polymer from the melt, as it iscustomary for thermoplastics, it is necessary to deactivate thepolymerization initiator, to remove the adhering monomer residues fromthe polymer and to break down the unstable fractions.

Thus, it is known that the deactivation of the initiator is carried outin the aqueous phase or in an organic solvent, subsequent filtration,washing and drying steps being required. The deactivation of theinitiator with the addition of different deactivators can also beeffected in the melt (DE 3703790). The deactivation step is oftencarried out in combination with the demonomerization and the eliminationof unstable chain ends (DE 37 38 632 and EP 0 137 305). EP 0673 955describes a process in which crude polymer is treated with a steam whichalso contains small amounts of volatile base. In this way, unconvertedresidual monomer is removed and the initiator is deactivated. JP05059255 states that the initiator is deactivated by adding alkali metalor alkaline earth metal oxides to the polymer melt.

The elimination of unstable terminal groups, which usually remain in thecrude polymer after the polymerization and in particular lead to chaindegradation when the polymer is heated, is also a usual process step inthe preparation of POM copolymers. The unstable hemiacetal end groups intrioxane copolymers can be selectively broken down, for example, byhydrolysis, i.e. by treating the copolymer at temperatures of from 120to 220° C. with pressurized water comprising alkaline material,especially trialkylamines, and optionally with the addition of organicsolvents, especially lower alcohols, trioxane or dioxolane (KunststoffHandbuch, p. 316). After the hydrolysis, the polymer must beprecipitated again, washed and dried.

The object of the invention therefore is to develop a process whichmakes it possible economically to prepare stable copolymers of1,3,5-trioxane in a continuous process while avoiding the deficienciesof the known processes.

It has now been found that thermally stable POM polymers can be obtainedif the initiator, which in general is a strong protonic acid, is firstdissolved in a formaldehyde dialkylacetal, a substance which usually isknown to regulate the molecular weight of the POM polymer, and thenadded to the reaction mixture. The invention eliminates the need toutilize an organic solvent carrier, an unnecessary component in thereaction mechanism, for the introduction of the protonic or Lewis acidinto the reaction mixture.

The present invention accordingly relates to a process for preparingpolyoxymethylene copolymers, wherein 1,3,5-trioxane is polymerized withgenerally known comonomers in the presence of a strong protonic acidinitiator and in the presence of a formaldehyde dialkyl acetal, andwherein the initiator is dissolved in a portion of the formaldehydedialkyl acetal before admixing the same with trioxane and thecomonomers.

In the prior art process of the production of POM polymers, generallyformaldehyde dialkyl acetals are used as molecular weight regulators.Generally, the use of use a molecular weight regulator has not beenknown to produce high molecular weight polymers.

The advantage of the process according to the invention is, however,that through initial dissolution of the initiator in a formaldehydedialkylacetal it is possible to add a very low quantity and controlledamount of the initiator in a perfectly dispersed state to the monomermixture thereby controlling the reaction rate. Due to the very lowquantity of initiator in the reaction mixture it is possible to alsoproduce high molecular weight material although there is a small amountof molecular weight regulator present in the reaction mixture. Accordingto the invention, it is possible to avoid contamination of the monomersand resulting polymer with substances which are critical to thepolymerization process. For example, it is no longer necessary to add anagent to deactivate the initiator. In principle, it is also no longernecessary to perform hydrolysis to the crude polymer. However, tofurther reduce the content of unstable terminal groups in the polymer,it is advantageous to perform hydrolysis thereto.

In the process according to the invention, the initiator can bedissolved in a part of or in the total amount of formaldehyde dialkylacetal used. The formaldehyde dialkyl acetal comprising the dissolvedinitiator usually is added to the mixture of trioxane and comonomers,i.e. the reaction mixture. A further predetermined amount offormaldehyde dialkyl acetal can be directly added to the reactionmixture before or after admixing the formaldehyde dialkyl acetal anddissolved initiator solution therewith.

In another working example, the formaldehyde dialkyl acetal containingthe dissolved initiator is premixed with the comonomers before admixingthe same with the trioxane. Optionally, a further predetermined amountof formaldehyde dialkyl acetal may be added to the reaction mixtureafterwards.

In the process according to the invention strong protonic acids, inparticular heteropoly acids, perchloric acid and perfluoroalkanesulfonicacids, can be used as initiator. Trifluoromethanesulfonic acid is thepreferred initiator. The amount of the initiator generally is at leastabout 0.01 to about 1.0 ppm, based on the total amount of trioxane andcomonomers. Preferably the amount of the initiator is from about 0.03 toabout 0.4 ppm, and preferably from about 0.05 to about 0.2 ppm, based onthe total amount of trioxane and comonomers.

Suitable formaldehyde dialkyl acetals used according to the inventionare formaldehyde dimethyl acetal, formaldehyde diethyl acetal,formaldehyde dipropyl acetal, and formaldehyde dibutyl acetal.Formaldehyde dimethyl acetal, i.e. methylal, is preferred. The amount offormaldehyde dialkyl acetal, generally, is from about 3.4 to about 34mmol per total kg of trioxane and comonomers.

Suitable comonomers of the present invention are generally known and maybe selected from the group consisting of ethylene oxide, 1,3-dioxolane,1,3-trioxepane, diethylene glycol formal, 1,4-butanediol formal,1,3-dioxane, propylene oxide, trimethylene oxide, butadiene oxide,o-xylene glycol formal, thiodiglycol formal, 1,3-oxthiolane, andmixtures thereof. Particularly preferable comonomers are ethylene oxide,1,3-dioxolane, diethylene glycol formal, and 1,4-butanediol formal. Theamount of the comonomer utilized herein may range from about 0.2 toabout 10% by weight, preferably from about 0.4 to about 5% by weight,based on the total amount of trioxane and comonomers.

The polymerization process according to the invention can be performedin any polymerization reactor or combination of reactors known for theproduction of POM polymers.

Further, antioxidants, acid acceptors, lubricants, waxes, UVstabilizers, nitrogen-containing co-stabilizers and other products knownin the art for POM may be used as stabilizers and additives, eitherindividually or in combination.

All fillers and reinforcing materials customary and known for plastics,in particular polyacetal copolymers, may be used as fillers andreinforcing materials.

EXAMPLES Example 1

In a batch reactor operated at a temperature of about 80° C. and apressure of about 1 atms., 96.6% by weight of trioxane was mixed with3.4% by weight of dioxolane to form a monomer mixture. To this mixture0.2 ppm of trifluoromethanesulfonic acid (TFMSA) dissolved in 500 ppm offormaldehyde dimethyl acetal (Methylal) were added, the quantities inppm being based on the total weight of the monomer mixture. After aninduction period of about 30 seconds the polymerization started. Theobtained crude polymer was quenched in a water/triethylamine mixture andsubsequently hydrolyzed at 170° C. in a water/methanol (10/90) mixturefrom which it was precipitated at room temperature. From the driedproduct the melt viscosity ratio (MVR) value and, through themeasurement of the formaldehyde formation during 1 hour at 170° C. underalkaline conditions, the content of unstable terminal groups wasdetermined (for data, cf. Table 1).

Examples 2 and 3

The procedure in Example 1 was utilized herein and additional amounts ofmethylal were added to the monomer mixture. The MVR and percent ofunstablized terminal groups are shown (for data, cf. Table 1).

Comparative Examples 4 through 6

Utilizing the procedure of Example 1, 96.6% by weight of trioxane wasmixed with 3.4% by weight of dioxolane to form the monomer mixture. Tothis mixture 50 ppm of BF₃ gas and 0 ppm, 400 ppm or 1000 ppm offormaldehyde dimethyl acetal (Methylal) were added, respectively, to themonomer mixture of Examples 4,5 and 6, the quantities in ppm being basedon the total weight of the monomer mixture and being adjusted to obtainproducts having the same MVR values as in Examples 1-3, respectively.After an induction period of 30 seconds the polymerization started. Theobtained crude polymer was quenched in a water/triethylamine mixture andsubsequently hydrolyzed at 170° C. in a water/methanol (10/90) mixturefrom which it was precipitated at room temperature. The dried productwas analyzed as in Examples 1-3.

TABLE 1 TFMSA in Additional Total Unstable Thoxane Dioxolane BF₃Methylal Methylal Methylal MVR terminals Example % b.w. % b.w. ppmppm/ppm ppm ppm m/10 cm % 1 96.6 3.4 0.2/500 0 500 2.5 0.04 2 96.6 3.40.2/500 500 1000 9 0.035 3 96.6 3.4 0.2/500 1000 1500 27 0.03 4 96.6 3.450 0 0 2.5 0.25 5 96.6 3.4 50 400 400 9 0.20 6 96.6 3.4 50 1000 1000 270.18

In accordance with the data shown in Table 1, after the MVR values wereadjusted in Examples 3, 4, and 5 to be equal to those of Examples 1, 2and 3, the percentage of unstable terminal end groups of the polymerswere dramatically reduced (see Examples 1, 2 and 3) wherein smallamounts of trifluoromethanesulfonic acid dissolved in methylal wereadded to the reaction mixture.

1. A process for the preparation of polyoxymethylene copolymersexhibiting a reduced amount of unstable terminal end groups comprising,polymerizing 1,3,5-trioxane with at least one cyclic ether and or acetalcomonomer with the aid of a strong protonic acid or Lewis acid initiatorand in the presence of a formaldehyde dialkyl acetal, the improvementcomprising dissolving the initiator in the formaldehyde dialkyl acetalbefore introducing the same to the trioxane and the comonomers.
 2. Theprocess according to claim 1, wherein the strong protonic acid initiatoris selected from the group consisting of trifluoromethanesulfonic acidand anhydrides, pentafluoroethylsulfonic aicd and anhydrides,heptafluoropropylsulfonic acid and anhydrides, nonafluorobutyl sulfonicacid and anhydrides, and perfluoroheptylsulfonic acid, anhydrides, andmixutres thereof and the Lewis acid is selected from the groupconsisting of phosphorus pentafluoride, silicon tetrafluoride, borontrifluoride, boron trifluoride eatherates, tintetrachloride, arsenicpentafluoride, triphenylmethyl hexafluorophosphate, and mixturesthereof.
 3. The process according to claim 2, wherein the strongprotonic acid initiator is trifluoromethanesulfonic acid and the Lewisacid is boron trifluoride.
 4. The process according to claim 3, whereinthe strong protonic acid or Lewis acid initiator is present in an amountof from about 0.01 to about 1 ppm, based on the total amount of trioxaneand comonomers.
 5. The process according to claim 4, wherein theformaldehyde dialkyl acetal is selected from the group consisting offormaldehyde dimethyl acetal, formaldehyde diethyl acetal, formaldehydedipropyl acetal, formaldehyde dibutyl acetal, and mixtures thereof. 6.The process according to claim 5, wherein the formaldehyde dialkylacetal is formaldehyde dimethyl acetal.
 7. The process according toclaim 6, wherein the formaldehyde dialkyl acetal is present in an amountof from about 3.4 to about 34 mmol per kg of trioxane and comonomers. 8.The process according to claim 2, wherein the formaldehyde dialkylacetal containing the dissolved strong protonic acid initiator is addedto the comonomers before admixing to the trioxane.